Aita for telling my sister I'm not the mother of her children (Full Post)

Apparently, I should have said this sooner. She started yelling at our mom, saying I'm her favorite because if I wasn't, she would tell me to put my future aside and help her with her children. Our mom told her point-blank that she would never tell any of us to put our future on hold. She said she gave us all the same opportunity, and it's not anyone's fault she didn't use hers wisely. Now she has children, and I'm going to take my life more seriously. She needs to take responsibility for her children because no one told her to have them, and she can't look after them.

She started crying and left. Then my mom started yelling at me, saying she warned me from the very beginning, and I didn't listen. She said I need to stop being a pushover because I lost my whole childhood when I was supposed to be outside playing with friends, going out, and just being a kid. I was babysitting like I gave birth. She said it's my fault my sister became so entitled because when she made plans for me to go out, I would cancel to watch the children. She's not wrong; I would always feel sorry for my sister because she knew how to manipulate me just because I don't like to see people suffer or be sad.

Sorry for the long post; I just want to know if I'm the asshole for not putting my future on hold.

Update:
Thank you for all the support and kind words; it has been eye-opening. So, yesterday, after my sister left crying, she didn't come home immediately; she waited for her kids' school to be dismissed to pick them up. I know I should have expected her to try and poison them against me, but I really wanted to give her the benefit of the doubt that she wouldn't stoop that low.

When they came home, the children were crying; they ran up to me, asking me not to hate them - that broke me. My niece went into a panic attack because when she gets overwhelmed, she starts having panic attacks. I've brought her to a doctor before, and she was diagnosed with ADHD and anxiety, so it takes a lot of my energy to calm her down, as someone with ADHD as well.

I asked my mom to take the kids to my room, as that's the only place they feel safe when they're overwhelmed. After they were out of earshot, I went off on my sister; I became a little personal with some of the things I said, but here's the gist of it: "When will you grow up? You've had 7 to 8 years to run around, party, and do as you please while I've been raising your children to the point where they call me mom. When will you wake up and be the best mother to your children instead of going out drinking and jumping from one man to another? I'm sick and tired of you; I'm not putting my life on hold for you; I've done enough for you. I love the kids, but as the only sensible role model they have, I doing this to show them it's okay to leave people behind. So, get your shit together and be the best mother you can to those kids because they're amazing. Do you think dealing with two children who have ADHD and are on the spectrum is easy? It's not, but I did do it without any help from anyone, not even mom, because she was always working, and I can't really blame her because my dad passed, and she was the only one there to work and provide for my younger brother and me. So, yeah, I basically raised three kids from age 11 till now, and that's going to change."

I left her in the living room and called Tony and James, my niblings' father; I explained the situation, and I told them it's time for them to be a part of their children's lives, like they can't just send child support because children need their parents for proper development. Our 34-year-old older brother, Josh, called mom and told her to have me pack my bags and move in with him until my program started. I'm going to write letters for my niblings so their mom doesn't cause any more damage. I've made a list of things they like, how to get them to take their medicine, and how to calm them down; I even have a scrapbook of all our moments for each of them.